Let $S=\left\{0, \frac{1}{4}, \frac{1}{2}, \frac{3}{4}\right\}$. (a) Show that the sample vectors corresponding to the functions $1, \cos \pi x, \cos 2 \pi x$, and $\cos 3 \pi x$ form a basis for the vector space of all sample functions on S. (b) Write the sampled version of the function $f(x)=x$ in terms of this basis.